A new fossil, Liaoconodon hui, was found in China and has all three middle ear bones

Researchers
have discovered a complete mammalian fossil that includes a transitional
middle ear, which consists of three bones that paleontologists have been
searching for over 150 years.

Jin Meng,
study leader and curator in the Division of Paleontology at the American Museum
of Natural History, along with Wang Yuanqing and Li Chuankui, both from the
Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese
Academy of Sciences in Beijing, have found the first complete mammalian fossil
that includes the transitional middle ear.

Mammals
are defined as a class of air-breathing vertebrate animals that share
characteristics like hair and mammary glands in mothers with young. They also
share three middle ear bones called the malleus, incus and ectotympanic. Two of
these bones are found in the joint of the lower jaw in reptiles, and
researchers believe that an evolutionary shift from lizards to mammals
separated the quadrate and articular plus prearticular bones from the posterior
lower jaw, and they became associated with hearing as the malleus and the
incus.

Previous
fossils show early mammals with reptilian jaw joints and reductions in these bones for both chewing and hearing
while other early mammalian fossils have ossified cartilage still connected to
the groove on their lower jaws. But none of these fossils had the middle ear
bones, and more evidence was needed to confirm this early transition and the
mysteries of the mammalian middle ear.

"People
have been looking for this specimen for over 150 years since noticing a
puzzling groove on the lower jaw of some early mammals," said Meng.
"Now we have cartilage with ear bones attached, the first clear
paleontological evidence showing relationships between the lower jaw and middle
ear."

The new fossil, which is called Liaoconodon hui, is
a medium-sized mammal measuring 35.7 cm long. It dates from the Mesozoic (about
125 to 122 million years) and was named after the fossil beds in Liaoning,
China, which is where it was discovered. It was also named after Yaoming Hu,
who was a graduate of the American Museum of Natural History's doctoral program
and passed away recently.

Liaoconodon hui is complete, and shows researchers that the
incus and malleus are detached from the lower jaw in order to create part of the middle ear. According to the study, the incus
and the malleus "remain linked to the jaw by the ossified Meckel's
cartilage that rests in the groove on the lower jaw," and the eardrum was
stabilized with this cartilage as support.

"Before
we did not know the detailed morphology of how the bones of the middle ear
detached, or the purpose of the ossified cartilage," said Meng. "Liaoconodon
hui changes previous interpretations because we now know the detailed
morphology of the transitional mammals and can propose that the ossified
cartilage is a stabilizer."

This
study also found that the middle ear "probably" evolved twice in monotremes,
marsupials and placentals. This was determined by features associated with the
groove on the lower jaw and other bones, including the presence of ossified
Meckel's cartilage.